Bivalent histone modifications in stem cells poise miRNA loci for CpG island hypermethylation in human cancer

It has been proposed that the existence of stem cell epigenetic patterns confer a greater likelihood of CpG island hypermethylation on tumor suppressor-coding genes in cancer. The suggested mechanism is based on the Polycomb-mediated methylation of K27 of histone H3 and the recruitment of DNA methyltransferases on the promoters of tumor suppressor genes in cancer cells, when those genes are preferentially pre-marked in embryonic stem cells (ESCs) with bivalent chromatin domains. On the other hand, miRNAs appear to be dysregulated in cancer, with many studies reporting silencing of miRNA genes due to aberrant hypermethylation of their promoter regions. We wondered whether a pre-existing histone modification profile in stem cells might also contribute to the DNA methylation-associated silencing of miRNA genes in cancer. To address this, we examined a group of tumor suppressor miRNA genes previously reported to become hypermethylated and inactivated specifically in cancer cells. We analyzed the epigenetic events that take place along their promoters in human embryonic stem cells and in transformed cells. Our results suggest that there is a positive correlation between the existence of bivalent chromatin domains on miRNA promoters in ESCs and the hypermethylation of those genes in cancer, leading us to conclude that this epigenetic mark could be a mechanism that prepares miRNA promoters for further DNA hypermethylation in human tumors.